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EXCERPT page containing first few paragraphs. 2023-02-04 08:21:46

MTF Measurement vs Field Curvature

There is no choice with field curvature— the optical design is fixed.

Stopping down can mitigate the impact of field curvature, but otherwise all the photographer can do is to understand the behavior, and sometimes exploit it to advantage, while avoiding compositions that are in conflict with the shape of the curvature.

When reading MTF charts, the dips and humps seen with some designs, particularly those from Zeiss and Leica, are typically due to field curvature—see Graphs 1a and 1b. In fact, some Leica designs for wide angles have very wavy MTF curves showing both strong astigmatism and field curvature; the Leica Super-Elmarit-R 15 mm f/2.8 ASPH is one example.

The best focus overall for across-the-frame sharpness is a compromise; adjusting focus for a neutral middle ground, then stopping down, can yield the best results (keep this in mind next time you read simplistic rules about depth of field).

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Diglloyd Making Sharp Images articulates years of best practices and how-to, painstakingly learned over a decade of camera and lens evaluation.

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  • Eases into photographic challenges with an introductory section.
  • Covers aspects of digital sensor technology that relate to getting the best image quality.
  • Technique section discusses every aspect of making a sharp image handheld or on a tripod.
  • Depth of field and how to bypass depth of field limitations via focus stacking.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
  • Optical aberrations: what they are, what they look like, and what to do about them.
  • How to test a lens for a “bad sample”.

Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.

Graphs at f/4 for the same lens
One focused optimally for f/4 and the other for f/1.4

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